The present invention relates to an on-chip source follower amplifier for a solid-state image sensing device output portion.
In a conventional on-chip source follower amplifier for a solid-state image sensing device output portion, an increase in gain is realized by a technique disclosed in Japanese Patent Laid-Open No 60-223161. FIG. 8 shows the structure of the on-chip source follower amplifier for a solid-state image sensing device, which is disclosed in Japanese Patent Laid-Open No 60-223161.
Referring to FIG. 8, on the surface of a semiconductor substrate 14 having one conductivity type, conductive regions 15 having a conductivity type opposite to that of the semiconductor substrate 14 and an opposite conductivity-type conductive region 16 independently of the opposite conductivity-type conductive region 15 are formed. A driver transistor 12 is formed in the opposite conductivity-type conductive region 16. Devices including a load transistor 13 other than the driver transistor 12 are formed in the opposite conductivity-type conductive regions 15.
The driver transistor 12 has a drain 17a and a source 18a, which are separately formed on the surface of the opposite conductivity-type conductive region 16, and a gate 19a formed on the opposite conductivity-type conductive region 16 between the drain 17a and the source 18a via a gate insulating film. The load transistor 13 has a drain 17a and a source 18b, which are separately formed on the surface of the opposite conductivity-type conductive region 15, and a gate 19b formed on the opposite conductivity-type conductive region 15 between the drain 17b and the source 18b via the gate insulating film.
FIG. 7 shows an equivalent circuit of the solid-state image sensing device output portion shown in FIG. 8. Referring to FIG. 7, the gate of the driver transistor 12 is connected to an input terminal 1, the drain is connected to a power supply terminal 2, and the source is connected to an output terminal 3. The gate of the load transistor 13 is connected to a bias power supply 24, the drain is connected to the source of the driver transistor 12, and the source is grounded.
In this arrangement, letting gm be the transconductance of the driver transistor 12, gmb be the conductance of the opposite conductivity-type conductive region (to be referred to as a back gate hereinafter) 16, gds1 be the output conductance of the driver transistor 12, and gds2 be the output conductance of the load transistor 13, a gain G of the source follower amplifier is given by: EQU G=gm/(gm+gmb+gds1+gds2) (1)
For this reason, when the back gate 16 is connected to the source 18a of the driver transistor 12, as in the prior art, gmb=0, and the gain G can be increased.
In the solid-state image sensing device, generally, the semiconductor substrate 14 is of an n type, and the opposite conductivity-type conductive regions 15 and 16 are of a p type. The power supply voltage 2 of the source follower amplifier, which is supplied to the power supply terminal 2, is 15 V. Conventionally, a voltage of about 15 V is applied to the semiconductor substrate 14. In fact, however, a voltage of about 5 V is applied along with the decrease in voltage. The opposite conductivity-type conductive region 15 is set at 0 V. The opposite conductivity-type conductive region 16 is set at about 8 V because it is connected to the source 18a of the driver transistor.
No problems are posed when the voltage of the semiconductor substrate 14 is 15 V, as in the prior art. However, when the voltage lowers to about 5 V, the p-type opposite conductivity-type conductive region 16 and the n-type semiconductor substrate 14 are forward-biased, so the function of the device is lost.